1. Field of the Invention
This invention relates to a fuel feed pump, and more particularly to a fuel feed pump of a frictional pump type, e.g. one effectively used for an electronically controlled fuel injection device.
2. Related Art Statement
Since a frictional pump used in a fuel feed pump has no self-priming properties in general, as the vapor quantity in a casing increases, discharge quantity decreases, thus resulting in a vapor lock.
Therefore, it has been the common practice that, in the conventional fuel feed pump of this type, deairing holes are opened in a generally annular groove path formed in an annular zone between an intake port and a discharge port except a liquid sealing portion in a casing, whereby vapor is deaired through the deairing holes when the fuel is separated from the vapor by the centrifugal force.
The reason why the deairing holes have heretofore been downwardly opened resides in that, in order to wash and cool a commutator and brushes in a motor portion provided in the upper portion of the fuel feed pump, the interior of a housing of the motor portion is made to serve as a fuel path, whereby the interior of the housing of the motor portion is highly pressurized, therefore the vapor cannot be deaired through the deairing holes, so that the deairing holes are opened downwardly in order that the vapor does not intrude into the interior of the housing where the motor portion is disposed.
However, recently, the number of component parts housed in an engine spaces of a motor vehicle has greatly increased, whereby, with the fuel feed pump in which only the downwardly opened deairing holes are formed in the intermediate portion of the groove path, the deairing effect has become insufficient.
Namely, as the number of the component parts in the engine space of the motor vehicle is increased highly, the temperature in the engine space tends to be raised. In an electronically controlled fuel injection device, a part of the fuel constantly circulates through the engine space and returns to a fuel tank, whereby the temperature of the fuel in the tank tends to further easily increase. As the temperature of the fuel is raised, boiling of the fuel itself and agitation of the fuel by the fuel feed pump produce a large quantity of vapor. When the large quantity of vapor is produced, the arrangement in which only the deairing holes are opened in the intermediate portion of the groove path can not deair the vapor sufficiently. Particularly, the vapor is lower in gravity than the fuel, whereby the vapor tends to accumulate in the upper space of the groove path, so that, in the conventional fuel feed pump in which the deairing holes are opened downwardly, the vapor cannot be deaired sufficiently.
Then, even if it is tried to deair the vapor into the interior of the motor portion from the upper space of the groove path, the vapor cannot be discharged upwardly because the vapor discharged from the deairing holes is lower in pressure than the highly pressurized fuel passing through the interior of the motor portion.
Further, when the highly pressurized fuel and the vapor which has not been deaired return to the intake side, the pressure is abruptly lowered, whereby intake of the fuel through the intake port is disturbed by the new production of the vapor and the expansion of the vapor, so that the vapor lock tends to occur.